White light emitting OLEDs from combined monomer and aggregate emission

ABSTRACT

The present invention relates to efficient organic light emitting devices (OLEDs). More specifically, the present invention relates to white-emitting OLEDs, or WOLEDs. The devices of the present invention employ two emitters in a single emissive region to sufficiently cover the visible spectrum. White emission is achieved from two emitters in a single emissive region through the formation of an aggregate by one of the emissive centers. This allows the construction of simple, bright and efficient WOLEDs that exhibit a high color rendering index.

This application is a continuation-in-part of U.S. patent application,Ser. No. 10/112,257, filed Mar. 29, 2002, which claims the benefit under35 U.S.C. §119(e) of provisional application Ser. No. 60/344,133, filedDec. 28, 2001, the contents all of which are incorporated herein byreference. This application also claims the benefit under 35 U.S.C.§119(e) of provisional application Ser. No. 60/368,496, filed Mar. 29,2002.

RESEARCH AGREEMENTS

The claimed invention was made by, on behalf of, and/or in connectionwith one or more of the following parties to a jointuniversity-corporation research agreement: Princeton University, TheUniversity of Southern California, and the University DisplayCorporation. The agreement was in effect on and before the date theclaimed invention was made, and the claimed invention was made as aresult of activities undertaken within the scope of the agreement.

GOVERNMENT RIGHTS

This invention was made with Government support under Contract No.F33615-94-1-1414 awarded by DARPA. The government has certain rights inthis invention.

FIELD OF THE INVENTION

The present invention relates to efficient organic light emittingdevices (OLEDs). More specifically, the present invention relates towhite-emitting OLEDs, or WOLEDs. The devices of the present inventionemploy two emitters in a single emissive region to sufficiently coverthe visible spectrum. White emission is achieved from two emitters in asingle emissive region through the formation of an aggregate by one ofthe emissive centers. This allows the construction of simple, bright andefficient WOLEDs that exhibit a high color rendering index.

BACKGROUND OF THE INVENTION

Organic light emitting devices (OLEDs), which make use of thin filmmaterials that emit light when excited by electric current, are expectedto become an increasingly popular form of flat panel display technology.This is because OLEDs have a wide variety of potential applications,including cell phones, personal digital assistants (PDAs), computerdisplays, informational displays in vehicles, television monitors, aswell as light sources for general illumination. Due to their brightcolors, wide viewing angle, compatibility with full motion video, broadtemperature ranges, thin and conformable form factor, low powerrequirements and the potential for low cost manufacturing processes,OLEDs are seen as a future replacement technology for cathode ray tubes(CRTs) and liquid crystal displays (LCDs), which currently dominate thegrowing $40 billion annual electronic display market. Due to their highluminous efficiencies, electrophosphorescent OLEDs are seen as havingthe potential to replace incandescent, and perhaps even fluorescent,lamps for certain types of applications.

Devices whose structure is based upon the use of layers of organicoptoelectronic materials generally rely on a common mechanism leading tooptical emission. Typically, this mechanism is based upon the radiativerecombination of a trapped charge. Specifically, OLEDs are comprised ofat least two thin organic layers between an anode and a cathode. Thematerial of one of these layers is specifically chosen based on thematerial's ability to transport holes, a “hole transporting layer”(HTL), and the material of the other layer is specifically selectedaccording to its ability to transport electrons, an “electrontransporting layer” (ETL). With such a construction, the device can beviewed as a diode with a forward bias when the potential applied to theanode is higher than the potential applied to the cathode. Under thesebias conditions, the anode injects holes (positive charge carriers) intothe HTL, while the cathode injects electrons into the ETL. The portionof the luminescent medium adjacent to the anode thus forms a holeinjecting and transporting zone, while the portion of the luminescentmedium adjacent to the cathode forms an electron injecting andtransporting zone. The injected holes and electrons each migrate towardthe oppositely charged electrode. When an electron and hole localize onthe same molecule, a Frenkel exciton is formed. These excitons aretrapped in the material which has the lowest HOMO-LUMO energy gap.Recombination of the short-lived excitons may be visualized as anelectron dropping from the lowest unoccupied molecular orbital (LUMO) tothe highest occupied molecular orbital (HOMO), with relaxationoccurring, under certain conditions, preferentially via a photoemissivemechanism.

The materials that function as the ETL or HTL of an OLED may also serveas the medium in which exciton formation and electroluminescent emissionoccur. Such OLEDs are referred to as having a “single heterostructure”(SH). Alternatively, the electroluminescent material may be present in aseparate emissive layer between the HTL and the ETL in what is referredto as a “double heterostructure” (DH).

In a single heterostructure OLED, either holes are injected from the HTLinto the ETL where they combine with electrons to form excitons, orelectrons are injected from the ETL into the HTL where they combine withholes to form excitons. Because excitons are trapped in the materialhaving the lowest energy gap, and commonly used ETL materials generallyhave smaller energy gaps than commonly used HTL materials, the emissivelayer of a single heterostructure device is typically the ETL. In suchan OLED, the materials used for the ETL and HTL should be chosen suchthat holes can be injected efficiently from the HTL into the ETL. Also,the best OLEDs are believed to have good energy level alignment betweenthe HOMO levels of the HTL and ETL materials.

In a double heterostructure OLED, holes are injected from the HTL andelectrons are injected from the ETL into the separate emissive layer,where the holes and electrons combine to form excitons.

Light emission from OLEDs has typically been via fluorescence, howeverOLED emission via phosphorescence has been recently demonstrated. Asused herein, the term “phosphorescence” refers to emission from atriplet excited state of an organic molecule and the term “fluorescence”refers to emission from a singlet excited state of an organic molecule.The term luminescence refers to either fluorescent or phosphorescentemission.

Successful utilization of phosphorescence holds enormous promise fororganicelectroluminescent devices. For example, an advantage ofphosphorescence is that potentially all excitons formed by therecombination of holes and electrons, either as a singlet or tripletexcited state, may participate in luminescence. This is because thelowest singlet excited state of an organic molecule is typically at aslightly higher energy than the lowest triplet excited state. Forexample, in typical phosphorescent organometallic compounds, the lowestsinglet excited state may rapidly decay to the lowest triplet excitedstate, from which the phosphorescence is produced. In contrast, only asmall percentage (about 25%) of excitons in fluorescent devices arecapable of producing the fluorescent luminescence that is obtained froma singlet excited state. The remaining excitons in a fluorescent device,which are produced in the lowest triplet excited state, are typicallynot capable of being converted into the higher energy singlet excitedstates from which the fluorescence is produced. This energy, thus,becomes lost to decay processes that heat-up the device rather than emitvisible light.

Typically, phosphorescent emission from organic molecules is less commonthan fluorescent emission. However, phosphorescence can be observed fromorganic molecules under an appropriate set of conditions. Organicmolecules coordinated to lanthanide elements often emit from excitedstates localized on the lanthanide metal. Such radiative emission is notfrom a triplet excited state. Furthermore, such emission has not beenshown to be capable of producing efficiencies high enough to be ofpractical value in anticipated OLED applications. The europiumdiketonate complexes illustrate one group of these types of species.

Organic phosphorescence may be observed in molecules containingheteroatoms with unshared pairs of electrons but, typically, only atvery low temperatures. Benzophenone and 2,2′-bipyridine are suchmolecules. Phosphorescence can be enhanced over fluorescence at roomtemperature by confining, preferably through bonding, the organicmolecule in close proximity to an atom of high atomic number. Thisphenomenon, called the heavy atom effect, is created by a mechanismknown as spin-orbit coupling. A related phosphorescent transition is ametal-to-ligand charge transfer (MLCT) that is observed in moleculessuch as tris(2-phenylpyridine)iridium(III).

The realization of highly efficient blue, green and redelectrophosphorescence is a requirement for full color displayapplications with low power consumption. Recently, high-efficiency greenand red organic electrophosphorescent devices have been demonstratedwhich harvest both singlet and triplet excitons, leading to internalquantum efficiencies (η_(int)) approaching 100%. See Baldo, M. A.,O'Brien, D. F., You, Y., Shoustikov, A., Sibley, S., Thompson, M. E.,and Forrest, S. R., Nature (London), 395,151-154 (1998); Baldo, M. A.,Lamansky, S., Burrows, P. E., Thompson, M. E., and Forrest, S. R., Appl.Phys. Lett., 75, 4-6 (1999); Adachi, C., Baldo, M. A., and Forrest, S.R., App. Phys. Lett., 77, 904-906, (2000); Adachi, C., Lamansky, S.,Baldo, M. A., Kwong, R. C., Thompson, M. E., and Forrest, S. R., App.Phys. Lett., 78, 1622-1624 (2001); and Adachi, C., Baldo, M. A.,Thompson, M. E., and Forrest, S. R., Bull. Am. Phys. Soc., 46, 863(2001). Using a green phosphorescent material, factris(2-phenylpyridine)iridium (Ir(ppy)₃), in particular, an externalquantum efficiency (η_(ext)) of (17.6±0.5)% corresponding to an internalquantum efficiency of >85%, was realized using a wide energy gap hostmaterial, 3-phenyl-4-(1′-naphthyl)-5-phenyl-1,2,4-triazole (TAZ). SeeAdachi, C., Baldo, M. A., Thompson, M. E., and Forrest, S. R., Bull. Am.Phys. Soc., 46, 863 (2001). More recently, high-efficiency(η_(ext)=(7.0±0.5)%) red electrophosphorescence was demonstratedemploying bis(2-(2′-benzo[4,5-a]thienyl)pyridinato-N, C³)iridium(acetylacetonate)[Btp₂Ir(acac)]. See Adachi, C., Lamansky, S., Baldo, M.A., Kwong, R. C., Thompson, M. E., and Forrest, S. R., App. Phys. Lett.,78, 1622-1624 (2001).

In each of these latter cases, high efficiencies are obtained by energytransfer from both the host singlet and triplet states to the phosphortriplet, or via direct trapping of charge on the phosphorescentmaterial, thereby harvesting up to 100% of the excited states. This is asignificant improvement over what can be expected using fluorescence ineither small molecule or polymer organic light emitting devices (OLEDs).See Baldo, M. A., O'Brien, D. F., Thompson, M. E., and Forrest, S. R.,Phys. Rev., B 60, 14422-14428 (1999); Friend, R. H., Gymer, R. W.,Holmes, A. B., Burroughes, J. H., Marks, R. N., Taliani, C., Bradley, D.D. C., Dos Santos, D. A., Bredas, J. L., Logdlund, M., Salaneck, W. R.,Nature (London), 397, 121-128 (1999); and Cao, Y, Parker, 1. D., Yu, G.,Zhang, C., and Heeger, A. J., Nature (London), 397, 414-417 (1999). Ineither case, these transfers entail a resonant, exothermic process. Asthe triplet energy of the phosphorescent material increases, it becomesless likely to find an appropriate host with a suitably high energytriplet state. See Baldo, M. A., and Forrest, S. R., Phys. Rev. B62,10958-10966 (2000). The very large excitonic energies required of thehost also suggest that the host material may not have appropriate energylevel alignments with other materials used in an OLED structure, henceresulting in a further reduction in efficiency. To eliminate thiscompetition between the conductive and energy transfer properties of thehost, a route to efficient blue electrophosphorescence may involve theendothermic energy transfer from a near resonant excited state of thehost to the higher triplet energy of the phosphorescent material. SeeBaldo, M. A., and Forrest, S. R., Phys. Rev. B 62,10958-10966 (2000);Ford, W. E., Rodgers, M. A. J., J. Phys. Chem., 96, 2917-2920 (1992);and Harriman, A.; Hissler, M.; Khatyr, A.; Ziessel, R. Chem. Commun.,735-736 (1999). Provided that the energy required in the transfer is notsignificantly greater than the thermal energy, this process may be veryefficient.

The quality of white illumination sources can be fully described by asimple set of parameters. The color of the light source is given by itsCIE chromaticity coordinates x and y. The CIE coordinates are typicallyrepresented on a two dimensional plot. Monochromatic colors fall on theperimeter of the horseshoe shaped curve starting with blue in the lowerleft, running through the colors of the spectrum in a clockwisedirection to red in the lower right. The CIE coordinates of a lightsource of given energy and spectral shape will fall within the area ofthe curve. Summing light at all wavelengths uniformly gives the white orneutral point, found at the center of the diagram (CIE x,y-coordinates,0.33, 0.33). Mixing light from two or more sources gives light whosecolor is represented by the intensity weighted average of the CIEcoordinates of the independent sources. Thus, mixing light from two ormore sources can be used to generate white light. While the twocomponent and three component white light sources will appear identicalto an observer (CIE x,y-coordinates, 0.32, 0.32), they will not beequivalent illumination sources. When considering the use of these whitelight sources for illumination, the CIE color rendering index (CRI)needs to be considered in addition to the CIE coordinates of the source.The CRI gives an indication of how well the light source will rendercolors of objects it illuminates. A perfect match of a given source tothe standard illuminant gives a CRI of 100. Though a CRI value of atleast 70 may be acceptable for certain applications, a preferred whitelight source will have a CRI of about 80 or higher.

The most successful approaches used to generate white OLEDs describedpreviously involve separating three different emitters (luminescentdopants) into individual layers. Three emissive centers are needed toachieve good color rendering index (CRI) values, as the lines aretypically not broad enough to cover the entire visible spectrum withfewer than three emitters. One approach to the design of WOLEDs involvessegregating the individual dopants into separate layers. The emissivezone in such a device is thus composed of distinct emissive layers.Kido, J. et. al. Science, 267, 1332-1334 (1995). The design of such adevice can be complicated, since careful control of the thickness andcomposition of each layer is critical for achieving good color balance.The separation of emitters into individual layers is essential toprevent energy transfer between the red, green and blue emitters. Theproblem is that the highest energy emitter (blue) will efficientlytransfer its exciton to the green and red emitters. The efficiency ofthis energy transfer process is described by the Forster energy transferequations. If the blue emitter has good spectral overlap with theabsorption spectra of the green or red emitters, and the oscillatorstrengths are high for all of the transitions, the energy transferprocess will be efficient. These energy transfers can occur overdistances of 30 Å or more. Likewise the green emitter will readilytransfer its exciton to the red emitter. The end result is that the redemitter dominates the spectral composition if the three are doped intothe film at equal concentrations. With fluorescent dyes the excitonmigration lengths are comparatively short and the balance between thethree emission colors can be controlled by varying the dopant ratios(more blue is needed than green and more green than red to achieve equalintensity at all three colors). If the dopant concentration is kept lowthe average distance between dopants can be kept below the Forsterenergy transfer distance and the affects of energy transfer can beminimized. Having all three dyes within a single layer involves a fourcomponent film, with each dopant present at <1%. The preparation of sucha film is difficult to carry-out reliably. Any shift in dopant ratiowill severely affect the color quality of the device.

The situation with phosphorescent emitters is somewhat different. Whilethe Forster radii of phosphorescent dopants may be lower than forfluorescent dopants, the exciton diffusion lengths can be >1000 Å. Inorder to achieve high efficiency with electrophosphorescent devices, thephosphorescent materials generally need to be present at much higherconcentrations than for fluorescent dopants (typically >6%). The endresult is that mixing the phosphorescent materials together in a singlelayer leads to a severe energy transfer problem, just as observed forfluorescent emitters. The approach that has been used successfullysegregates the phosphorescent materials into separate layers,eliminating the energy transfer problem.

SUMMARY OF THE INVENTION

The present invention is directed to efficient organic light emittingdevices (OLEDs). More specifically, the present invention is directed towhite-emitting OLEDs, or WOLEDs. The devices of the present inventionemploy two luminescent emitters, or lumophores, in a single emissiveregion to sufficiently cover the visible spectrum. The lumophores mayemit via fluorescence (from singlet excited states) or viaphosphorescence (from triplet excited states). White emission isachieved from two luminescent emitters in a single emissive regionthrough the formation of an aggregate by one of the lumophores. The twoemissive centers (the aggregate emitter and the monomer emitter) aredoped into a single emissive layer. This allows the construction ofsimple, bright and efficient WOLEDs that exhibit a high color renderingindex.

Thus, an object of the present invention is to produce white lightemitting OLEDs that exhibit high external emission efficiency (η_(ext))and luminance.

Another object of the invention is to produce white-light-emitting OLEDsthat exhibit a high color rendering index.

Yet another object of the invention is to produce white-light-emittingorganic light emitting devices that produce white emission having CIEx,y-chromaticity coordinates approaching (0.33, 0.33).

Yet another object of the invention is to provide OLEDs that may be usedfor large area, efficient light sources in diffuse-illuminationapplications, such as are now ubiquitously filled with conventionalfluorescent lamps.

For example, an object of the invention is to produce white lightemitting OLEDs comprising an emissive region, wherein the emissiveregion comprises an aggregate emitter, and a monomer emitter, whereinthe emission from the aggregate emitter is lower in energy than theemission from the monomer emitter, and wherein the combined emissionspectrum of the aggregate emitter and the monomer emitter sufficientlyspans the visible spectrum to give a white emission.

BRIEF DESCRIPTION OF THE FIGURES

For the purpose of illustrating the invention, representativeembodiments are shown in the accompanying figures, it being understoodthat the invention is not intended to be limited to the precisearrangements and instrumentalities shown.

FIG. 1 shows the electroluminescence spectra of FPt in TAZ (exciplex)and FIrpic in CBP (dopant only), along with the sum of the two spectra.The summed spectrum is not a single device, but arithmetically combinesthe output from two devices to illustrate the potential for usingmonomer and exciplex emitter to achieve true white emission.

FIG. 2 shows the photoluminescence spectra of CBP films doped with <1%and >6% FPt. At 1% loading, the spectrum is dominated by monomeremission. At 6% loading, the monomer signal is still present, but is aminor component relative to the yellow, excimer emission.

FIG. 3 shows the EL spectra at several different current levels for thedevice ITO/PEDOT(400)/NPD(300)/CBP:FIrpic 6%:FPt6%(300)/FIrpic(500)/LiF(5)/Al(1000). The CIE coordinates for each of thespectra are given in the legend. The device appears white at all currentdensities, with CRI values as high as 70.

FIG. 4 shows the quantum efficiency (open circles) and power efficiency(open squares) plot for the device ITO/PEDOT(400)/NPD(300)/CBP:FIrpic6%:FPt 6%(300)/FIrpic(500)/LiF(5)/Al(1000). The current density-voltageplot is shown in the insert. This device demonstrates that excimeremission can be coupled with monomer emission in a single OLED toachieve white light emission.

FIG. 5 shows the photoluminescence emission (solid lines) and excitationspectra (open circles) of Films 1-4. The films have a thickness of 1000Å and are grown on quartz substrate. Film 1 shows the CBP PL spectrumwith a peak at λ=390 nm and the corresponding PLE between λ=220- and 370nm. The PLE of CBP has a shoulder at λ=300 nm and a main peak at λ=350nm. The CBP PLE peaks correspond to the absorption peaks at λ=300- and350 nm (arrows, inset of FIG. 6). These two CBP features appear in thePLE spectra of all films where CBP was used as a host; therefore, it isthe main absorbing species in all the films, and energy must betransferring efficiently from CBP to both FPt(acac) and FIr(pic) foremission from these molecules to occur.

FIG. 6A shows the normalized electroluminescent spectra of the deviceITO/PEDOT-PSS/NPD(30 nm)/CBP:FIrpic 6%:FPt 6%(30 nm)/BCP(50 nm)/LiF atseveral current densities, staggered vertically for viewing clarity. Theupper left inset shows the absorbance versus wavelength of a 1000 Åthick CBP film on quartz. FIG. 6B depicts the structure of the device.

FIG. 7 shows the plots of external quantum and power efficiencies versuscurrent density of the device ITO/PEDOT-PSS/NPD(30 nm)/CBP:FIrpic 6%:FPt6%(30 nm)/BCP(50 nm)/LiF. The emissive layer consists of 6 wt % FIr(pic)and 6 wt % FPt(acac) doped into CBP. The left Inset shows thecurrent-density versus voltage characteristics for this device. Theright inset depicts the energy level diagram of CBP, shown with solidlines, doped with FIr(pic) and FPt(acac) (dashed lines). Here, HOMOindicates the position of the highest occupied molecular orbital andLUMO indicates the position of the lowest unoccupied molecular orbital.

FIG. 8 shows the photoluminescence spectra of CBP films doped withvarying levels of FPt.

FIG. 9 shows the photoluminescence spectra of CBP films doped withvarying levels of FPt2.

FIG. 10 shows the photoluminescence spectra of CBP films doped withvarying levels of FPt3.

FIG. 11 shows the photoluminescence spectra of CBP films doped withvarying levels of FPt4.

FIG. 12 shows the plot of current density vs. voltage for the FPt3 dopedOLEDs: ITO/NPD (400 Å)/Ir(ppz)3 (200 Å)/CBP-FPt3 (300 Å)/BCP (150Å)/Alq3 (200 Å)/Mg—Ag with various concentrations of FPt3.

FIG. 13 shows the electroluminescent spectra of the FPt3 doped OLEDs ofFIG. 12 at various concentrations of FPt3.

FIG. 14 shows CIE coordinates for the FPt3 doped OLEDs of FIG. 12 atvarious concentrations of FPt3.

FIG. 15 shows the plot of brightness vs. voltage for the FPt3 dopedOLEDs of FIG. 12 at various concentrations of FPt3.

FIG. 16 shows the plot of quantum efficiency vs. current density for theFPt3 doped OLEDs of FIG. 12 at various concentrations of FPt3.

FIG. 17 shows the power efficiency and brightness as a function ofcurrent density for the FPt3 doped OLEDs of FIG. 12 at variousconcentrations of FPt3.

FIG. 18 shows the chemical structures of the compounds platinum(II)(2-(4′,6′-difluorophenyl)pyridinato-N,C^(2′)) (2,4-pentanedionato) (FPt,FPt(acac)), platinum(II) (2-(4′,6′-difluorophenyl)pyridinato-N,C^(2′))(2,2,6,6-tetramethyl-3,5-heptanedionato) (FPt2), platinum(II)(2-(4′,6′-difluorophenyl)pyridinato-N,C^(2′))(6-methyl-2,4-heptanedionato) (FPt3), platinum(II)(2-(4′,6′-difluorophenyl)pyridinato-N,C^(2′))(3-ethyl-2,4-pentanedionato) (FPt4),iridium-bis(4,6,-F₂-phenyl-pyridinato-N,C^(2′))-picolinate(FIrpic),fac-Iridium(III) tris(1-phenylpyrazolato-N,C^(2′)) (Irppz), andN,N′-meta-dicarbazoloylbenzene (mCP).

FIG. 19 shows the photoluminescence spectra of CBP films dopedseparately with 8% FPt (labeled “Me”) and 20% FPt2 (labeled “iPr”). Thegreater steric bulk of the FPt2 complex inhibits aggregate formation inCBP, in contrast to the 8% FPt doped CBP, which shows a broad aggregateemission.

FIG. 20 shows the photoluminescence spectra for varying concentrationsof FPt doped into an mCP thin film. The spectra were measured byexciting the film at the excitation maximum of the matrix material (300nm for mCP).

FIG. 21 shows the energy minimized structures of the CBP and mCP hostmolecules. The molecular modeling and energy minimization was carriedout at PM3 level using the MacSpartan Pro v 1.02 software package,Wavefunction Inc, Irvine, Calif. 92612.

FIG. 22 shows the energy level diagrams depicting the HOMO and LUMOlevels for selected materials. The energy for each orbital is listedbelow (HOMOs) or above (LUMOs) the appropriate bar. The HOMO and LUMOlevels for the emissive dopant FPt is shown as a dashed line in each ofthe plots. The doped luminescent layers (CBP or mCP) are enclosed inbracket. Each device had either a CBP or an mCP layer, not both. The topplot shows the diagram for a four layer OLED (no electron blockinglayer), and the bottom plot shows a similar OLED with an Irppz EBL.

FIG. 23 shows the device properties for a mCP based WOLED (ITO/NPD (400Å)/Irppz (200 Å)/mCP:FPt (doping level 16%, 300 Å)/BCP (150 Å)/Alq3 (200Å)/ LiF—Al). A schematic drawing of the device with the Irppz EBL isshown as an inset to the top plot. The spectra and CIE coordinates(inset) are shown in the top plot

FIG. 24 the quantum efficiency vs current density and current-voltagecharacteristics (inset) for the device depicted in FIG. 23.

FIG. 25 shows the Lumens per watt and brightness vs. current densityplots for the WOLED depicted in FIG. 23 and the related structurewithout the Irppz EBL.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention will now be described in detail for specificpreferred embodiments of the invention. These embodiments are intendedonly as illustrative examples and the invention is not to be limitedthereto.

The present invention is directed to producing efficient, white emittingOLEDs. The devices of the present invention employ two emitters in asingle emissive region to sufficiently cover the visible spectrum. Whiteemission is achieved from two luminescent emitters in a single emissiveregion through the formation of an aggregate by one of the lumophores.The aggregate emitter comprises two or more emissive molecules ormoieties that are bound in the ground state and/or in the excited state.Generally, the aggregate emitter will be composed of two emissivemolecules (i.e., a dimer), which may be alike or different. Thelumophores may emit via fluorescence (from singlet excited states) orvia phosphorescence (from triplet excited states). The two emissivecenters (the aggregate emitter and the monomer emitter) are doped into asingle emissive layer. This allows the construction of simple, brightand efficient WOLEDs that exhibit a high color rendering index. Amongmethods for producing white light, electrophosphorescence is preferredas the most effective mechanism for OLED light emission due to itsdemonstrated potential for achieving 100% internal quantum efficiency.Surprisingly, we have found that the phosphorescent emission from theaggregate excited state is often greater than would be expected, evengiven the potential for achieving 100% internal quantum efficiency.While electrophosphorescence, typically achieved by doping anorganometallic phosphorescent material into a conductive host, hassuccessfully been used to generate the primary colors necessary fordisplay applications, efficient generation of the broad spectralemission required of a white light source had, until now, remainedelusive.

While separating the emitting materials into separate layers may makethe color tuning of a three dopant electroluminescent devices arelatively straightforward process, such as disclosed in co-pendingapplication Ser. No. 60/291,496, entitled “High Efficiency Multi-colorElectro-phosphorescent OLEDs”, filed May 16, 2001, the multiple layerapproach adds complication to the device. It would be much simpler ifdevices could be made with two emitters rather than three and if the twoemitters could be present in the same emissive region. If that were thecase, devices might be fabricated in the same high efficiency, longlived structures that have been demonstrated for monochromaticelectroluminescent OLEDs. In order to use only two dopants and get anacceptable CRI value, one of the dopants must have a very broad emissionline. Unfortunately, broad emission lines had been typically observedonly in inefficient devices.

A promising approach to reducing the number of dopants and structuralheterogeneities inherent in the multiple color-band architecture is toemploy a lumophore that forms a broadly emitting exciplex (i.e. a statewhose wavefunction overlaps a neighboring, dissimilar molecule) in itsexcited state. Recently, fluorescent exciplex OLEDs have beendemonstrated with Commission Internationale de l'Eclairage (CIE)coordinates close to the ideal white light source of (0.33, 0.33), andan external efficiency of η_(ext)=0.3%, a luminance efficiency ofη_(p)=0.58 lm/W and a maximum luminance of 2000 cd/m². See Berggren, M.et al. J. Appl. Phys. 76, 7530-7534 (1994); and Feng, J. et al. ApplPhys. Lett. 78, 3947-3949 (2001). These values lie well below thoseneeded in practical lighting applications. The emission from such OLEDswas reported to be produced solely by exciplexes.

We have found that the solution to achieving an efficient white emittingOLED involves obtaining emission from two luminescent emitters in asingle emissive region, wherein one of the emissive centers is a monomerand the other emissive center is an aggregate. The aggregate emittercomprises two or more lunophores that are bound in the ground stateand/or in the excited state. Generally, the aggregate emitter will becomposed of two molecules (i.e., a dimer), which may be alike ordifferent.

An excimer or exciplex is formed when the lumophores comprising theaggregate emitter are bound in the excited state but not bound in theground state. An excimer is a dimer with an excited state wavefunctionthat extends over two identical molecules. An exciplex is a dimer withan excited state wavefunction that extends over two dissimilarmolecules. In excimers and exciplexes, the constituent molecules arebound together in the excited (excitonic) state, but rapidly dissociateto two discrete molecules after relaxation. The end result is that theexciton has no absorption in the ground state. Excimer and exciplexformation is favored where there is a significant overlap between theLUMOs of the constituent species. The excimer and exciplex energy islower than that of an exciton localized on either of the two moleculesthat make it up and it's emission is typically a broad line. Sinceexcimers and exciplexes both lack a bound ground state, they provide aunique solution to the achievement of efficient energy transfer from thecharge-carrying host matrix to the light emitting centers. Indeed, forthe case of two emitting centers, use of an excimer or exciplexprohibits energy transfer between the emitting centers, eliminatingcomplicated intermolecular interactions, which make color balancingusing multiple dopants problematic. For a review of the properties ofexcimers and excitons see Andrew Gilbert and Jim Baggott, Essentials ofMolecular Photochemistry, 1991, CRC Press, Boston, pp. 145-167.

In another embodiment of the invention, the molecules that comprise theaggregate emitter are bound in both the ground state and in the excitedstate. For example, a dimer of the phosphorescent organometalliccompound may have a metal-metal bond in the ground state. Practically,it can be difficult to determine whether the lumophores comprising theaggregate emitter are bound in the ground state or not, when doped intomolecular thin films, of the type used for the fabrication of OLEDs. Itmay be the case for some aggregate emitters that the truth is somewherebetween the extremes. For example, a dimer of the phosphorescentorganometallic compound may have a weak metal-metal bond in the groundstate, but in the excited state the bond shortens and the dimer becomesstrongly bound. In this case, the dimer is not an excimer or anexciplex, as the dimer is bound in the ground state. The phosphorescentdopants may well be involved in both π-π stacking and metal-metalinteractions in the doped films, leading to either excimer or MMLCTexcited states. Thus, emission from these films may have contributionsfrom both excimeric and oligomeric excited states. In either case, theemission spectra observed from the aggregate, whether bound in theground state or not, are typically broad and unstructured, occurring atlower energy than that of the monomer. Thus, the term “excimer” or“exciplex” as used herein may in some cases refer to aggregates havingstrongly bound excited states and weakly bound ground states.Additionally, the term “aggregate” as used herein includes excimers andexciplexes as conventionally understood.

In one embodiment of the invention, both monomer and aggregate emissionare achieved from the same dopant. Those dopant molecules that are inrelatively close contact with another dopant molecule will be able toform the aggregate state. Dopant molecules that are isolated will givemonomer and not aggregate emission. A white OLED may result, if therelative contribution from each emissive center is appropriatelycontrolled, for example, by adjusting the concentration of each emitterin the emissive layer. To achieve well balanced monomer and aggregateemission from an emissive layer with a single luminescent dopant andachieve high efficiencies the monomer-aggregate ratio must be achievedat an appropriate concentration of the dopant. Different approaches thataffect the nature of intermolecular interactions in the film, and thusthe degree of monomer-aggregate emission may be used to control themonomer-aggregate emission ratio. One such approach is to vary theamount of steric bulk in the dopant molecule. A second approach is tochange the host matrix. Both approaches are believed to affect thedegree of association of the dopant material in the emissive layer andhence the ratio of monomer and aggregate states.

Using such methods, it is a particular feature of the present inventionthat WOLEDs can be fabricated with both a very high efficiency and avery high CRI by adjusting the concentration of the dopant so as to besubstantially the same throughout the emissive layer. Naturallyoccurring variations in the distance between neighboring moleculesdetermines the extent of aggregate formation for a given host-dopantcombination, such that the desired balance of monomer and aggregateemission is achieved. The emission spectrum produced by the devices ofthe present invention sufficiently span the visible spectrum so as toappear is substantially white, for example, a CIE x-coordinate of about0.30 to about 0.40 in combination with a CIE y-coordinate of about 0.30to about 0.45. Preferably the CIE x,y-coordinates arc about (0.33,0.33). Moreover, the devices of the present invention are preferablycapable of producing white emission having a CIE (CommissionInternationale de l'Eclairage) color rendering index (CRI) of at leastabout 70. More preferably, the CRI is higher than about 80.Alternatively, instead of seeking a very high CRI, the method might beused to produce a selected color having prescribed CIE coordinates.

The devices of the present invention are comprised of an anode, an HTL,an ETL and a cathode. In addition, the device may contain additionallayers, such as, but not limited to, an exciton blocking layer (EBL), aseparate emissive layer, or a hole injection layer (HIL). In oneembodiment, the HTL will also serve as the region in which excitonformation and electroluminescent emission occur. Alternatively, the ETLmay serve as the region in which exciton formation andelectroluminescent emission occur. In yet another embodiment, the devicemay comprise a separate emissive layer in which exciton formation andelectroluminescent emission occur. The emissive region comprises twotypes of emissive centers. One of the emissive centers will form anaggregate to give a broad emission spectrum. The other emitter will emitas a monomer. In one embodiment of the invention, a single luminescentmaterial can emit as both the aggregate emitter and as the monomeremitter. Color optimization leading to a high color rendering index maybe achieved by selection of the aggregate forming emitter and themonomer emitter so that their emissions cover the visible spectrum andby varying the concentrations of the emitters.

In a preferred embodiment, the two luminescent materials are doped intoa matrix material. The matrix material will typically be a chargecarrying material. In a particularly preferred embodiment, theluminescent materials will be phosphorescent emitters (i.e., they willemit from triplet excited states). Materials that are present ascharge-carrying host and dopant are selected so as to have a high levelof energy transfer from the host to the dopant material. Highefficiencies are obtained by energy transfer from both the host singletand triplet states to the phosphor triplet, or via direct trapping ofcharge on the phosphorescent material, thereby harvesting up to 100% ofthe excited states. In addition, these materials need to be capable ofproducing acceptable electrical properties for the OLED. Furthermore,such host and dopant materials are preferably capable of beingincorporated into the OLED using starting materials that can be readilyincorporated into the OLED by using convenient fabrication techniques.For example, small molecule, non-polymeric materials, may be depositedby using line-of-sight vacuum-deposition techniques, or by using anorganic vapor phase deposition (OVPD) technique such as disclosed inapplication Ser. No. 08/972,156, filed Nov. 17, 1997, now U.S. Pat. No.6,337,102, and entitled “Low Pressure Vapor Phase Deposition Of OrganicThin Films”, which is incorporated herein in its entirety by reference.Alternatively, polymeric materials may be deposited using spin-coatingtechniques.

In a preferred embodiment of the present invention, the emissive regionof the device will be comprised of an excimer and a monomer emitter. Theluminescent materials that comprise the monomer and excimer may emit viafluorescence or phosphorescence. The monomer emitter will preferably beone that emits in the high energy (for example, blue or green) portionof the visible spectrum. The excimer emitter preferably provides a broademission which spans the low energy portion of the visible spectrum.There is no absorption into this excimer state, so there will be minimalenergy transfer from the monomeric emitter to the excimer.

To achieve efficient excimer emission from a doped layer, control of thedopant concentration is an important consideration. Excimers willtypically form when planar molecules are in close proximity to eachother. For example, square planar metal complexes, such as certaincomplexes of Pt, have been demonstrated to form excimers in concentratedsolutions and in thin films. For example, FPt(acac) shows monomeremission in 10⁻⁶ M solutions of dichloromethane and excimer emission inconcentrated 10⁻³ M solutions.

At appropriate concentrations, it is possible to get both monomer andexcimer emission from the same dopant. Only those dopant molecules thatare in relatively close contact with another dopant molecule will beable to form the excimer state. Dopant molecules that are isolated willgive monomer and not excimer emission. If the monomer is blue emittingand the excimer is yellow emitting, a white OLED may result, if therelative contribution from each emitter is appropriately controlled, forexample, by adjusting the concentration of each emitter in the emissivelayer.

Forming excimer states requires that the two emissive molecules be inclose proximity to each other, so that they can dimerize when one ofthem is promoted to its excited state. This suggests that there shouldbe a strong concentration dependence in excimer formation. For example,when FPt is doped into a host matrix, such as CBP, at 1%, only monomeremission is observed in the photoluminescence spectrum of the thin film.At this low doping level, the molecules are isolated. As the dopingconcentration is raised, the amount of excimer emission increases as themonomer line decreases. At FPt doping levels of 2-3%, the ratio ofmonomer to excimer emission is close to 1:1. At this doping level, someof the FPt molecules are isolated and others are in close proximity toanother FPt molecule, leading to efficient excimer formation. At adoping level of 6% nearly complete excimer emission is observed.

If an OLED is prepared with the FPt film doped at about 2-3%, it shouldbe possible to prepare a white OLED. This device has only a singledopant, present in a homogeneously doped thin film. Unfortunately, adoping level of about 2-3% is generally too low to efficiently quenchthe host excitons, resulting in significant host emission in theelectroluminescence spectrum. In order to make efficient phosphorescentOLEDs, the doping level is preferably above about 6%, with the highestefficiencies at higher doping levels. At doping levels high enough toprepare efficient OLEDs, only excimer emission is observed for FPt. Inorder to achieve more efficient white emission from a single dopant (viasimultaneous monomer and excimer emission) the doping level leading tobalanced emission should be increased.

When the steric bulk of the emitter is increased, the likelihood that anexcimer can be formed in the solid state is decreased. The added bulkprevents the molecules from associating closely. This is readily seen inthe spectra of FPt2 (FIG. 9). The t-Bu groups of the complex preventclose face-to-face packing of these molecules in the doped films. As aresult, only monomer emission is observed in the photoluminescencespectra of doped thin films of FPt2, at doping levels as high as 25%.FPt forms excimers too readily and FPt2 does not form excimers. Whenmolecules with intermediate levels of steric bulk are used, it ispossible to achieve mixed monomer/excimer emission at moderate dopinglevels. The role of is added steric bulk is clearly seen in thephotoluminescence spectra of thin films of FPt3 (FIG. 10) and Fpt4 (FIG.11). At lower doping levels, monomer emission is observed and at highdoping levels excimer emission is dominant. At close to 10% doping,closely balanced monomer-excimer emission for both of these complexes isobserved. At lower doping levels, a large contribution from the host maybe observed. As the doping level is increased the host contributiondecreases as expected for more efficient exciton transfer at higherdoping levels.

In another embodiment of the present experiment, the monomer/aggregateemission ratio is optimized by changing the host matrix material. Forexample, in a single dopant system (both monomer and aggregate emissionfrom the same dopant) the degree of association of the dopant in theemissive layer, and thus the ratio the emissive states, can be varied bymodifying the host matrix material. Without being limited by theory,during the growth of doped films, there are competing processes betweenthe aggregation of dopants and their dispersion in the host matrix. Ifthe host matrix acts as a good solvent, the dopants will be more evenlydispersed in the film, favoring monomeric species. A poorly solvatinghost matrix will not disperse the monomer dopant efficiently, leading todopant aggregation. Thus, a better solvating host will favor themonomeric species in comparison to the aggregate species at a givendopant concentration. Various properties of the host matrix material maybe important in determining its solvent properties for a particulardopant, including dipole moment, association energy and other physicalcharacteristics, such as degree of crystallinity.

In another representative embodiment, the device is prepared from twodifferent emissive materials so that one emits blue light from monomericexcited states. The other dopant emits from an excimeric state, leadingto broad yellow emission. For example, a device that was prepared todemonstrate this concept utilized a blue emissive octahedral Ir complex,FIrpic, which does not form excimeric states at any doping level. Theother dopant used in the device was a planar Pt complex, FPt, whichefficiently forms excimeric states, even at low doping levels. When eachof the dopants was present at 6%, the resulting electroluminescentemission consisted of roughly equal contributions from FIrpic (monomer)and FPt excimers.

Whenever an exciplex emitter is used to provide the broad emission whichspans the low energy portion of the visible spectrum, the emissiveregion of the device will be comprised of an exciplex and a monomeremitter. The luminescent materials that comprise the monomer and excimermay emit via fluorescence or phosphorescence. The monomer emitter willpreferably be one that emits in the high energy (for example, blue orgreen) portion of the visible spectrum. The exciplex emitter provides abroad emission which spans the low energy portion of the visiblespectrum. There is no absorption into this exciplex state, so there willbe minimal energy transfer from the monomeric emitter to the exciplex.Thus, if a device is prepared, for example, with two blue-emittingphosphorescent materials, one which forms an exciplex with the matrixmaterial and one which does not, a white device may be prepared. Theexciplex could emit yellow and would not trap energy from thenon-exciplex forming dopant, since the exciplex has no ground stateabsorption (the oscillator strength for absorption is zero, thus theForster radius=0). Thus, the ratio of blue to yellow emission may bereadily tuned by) varying the ratio of the two emitters, without thecomplication of energy transfer from the blue emitter to the yellow one.An example of two such materials are FIrpic and FPt. The Ir complex doesnot form exciplexes and the Pt complex forms a yellow exciplex in a TAZmatrix. The electroluminescence spectra of the two devices and their sumare shown in FIG. 1. The Ir based device has a peak external efficiencyof 6% and the Pt based device (exciplex emission) has an externalefficiency of 4%. The two light sources summed give a white light sourcewith a CRI of 82, comparable to some of the best illumination sources.

The monomer emitter, the emitter which is not involved in aggregateemission, may be chosen from among the high energy (e.g., blue)luminescent materials. The monomer emitter will typically be aluminescent compound with sufficient steric bulk to prevent the neededproximity in the solid state for the formation of aggregates at aparticular concentration. Preferred monomeric emitters compriseorganometallic transition metal complexes that have an octahedralcoordination geometry, or that have a square planar geometry and haveligands of sufficient steric bulk to prevent aggregate formation.

The phosphorescent materials for use in the present device are typicallyorgano-metallic compounds. The phosphorescent materials may be selectedfrom organometallic compounds as taught in co-pending applications U.S.Ser. No. 08/980,986, filed Jun. 18, 2001, now U.S. Pat. No. 6,303,238,and Ser. No. 09/978455, filed Oct. 16, 2001, each of which isincorporated herein in its entirety by reference.

Various compounds have been used as HTL materials or ETL materials. TheETL materials may include, in particular, an aryl-substitutedoxadiazole, an aryl-substituted triazole, an aryl-substitutedphenanthroline, a benzoxazole or a benzothiazole compound, for example,1,3-bis(N,N-t-butyl-phenyl)-1,3,4-oxadiazole (OXD-7),3-phenyl-4-(1′-naphthyl)-5-phenyl-1,2,4-triazole (TAZ);2,9-dimethyl-4,7-diphenyl-phenanthroline (bathocuproine or BCP);bis(2-(2-hydroxyphenyl)-bcnzoxazolate)zinc; orbis(2-(2-hydroxyphenyl)-benzothiazolate)zinc; such as disclosed in C.Adachi et al., Appl. Phys. Lett., vol. 77, 904 (2000). Other electrontransporting materials include (4-biphenyl)(4-tertbutylphenyl)oxadiazole(PDB) and aluminum tris(8-hydroxyquinolate) (Alq3).

The material of a hole transporting layer is selected to transport holesfrom an anode to an emission region of the device. HTL materials mostlyconsist of triaryl amines in various forms which show high holemobilities (˜10⁻³ cm²/Vs). An example of a material suitable as a holetransporting layer is 4,4′-bis[N-(naphthyl)-N-phenyl-amino]biphenyl(α-NPD) with a hole mobility of about 5×10⁻⁴ cm²/V sec. Other examplesincludeN,N′-bis(3-methylphenyl)-N,N′-diphenyl-[1,1′-biphenyl]4,4′-diamine(TPD), 4,4′-bis[N-(2-naphthyl)-N-phenyl-amino]biphenyl (β-NPD),4,4′-bis[N,N′-(3-tolyl)amino]-3,3′-dimethylbiphenyl (M14),4,4′,4′-tris(3-methylphenylphenylamino)triphenylamine (MTDATA),4,4′-bis[N,N′-(3-tolyl)amino]-3,3′-dimethylbiphenyl (HMTPD),3,3′-Dimethyl-N⁴,N⁴,N^(4′),N^(4′)-tetra-p-tolyl-biphenyl-4,4′-diamine(R854), N,N′,N″-1,3,5-tricarbazoloylbenzene (tCP) and4,4′-N,N′-dicarbazole-biphenyl (CBP). Additional suitable holetransporting materials are known in the art, and examples of materialsthat may be suitable for the hole transporting layer can be found inU.S. Pat. No. 5,707,745, which is incorporated herein by reference.

In addition to the small molecules discussed above, the matrix maycomprise a polymer or polymer blend. In one embodiment, the emissivematerial(s) are added as a free molecule, i.e. not bound to the polymer,but dissolved in a polymer “solvent”. A preferred polymer for use as amatrix material is poly(9-vinylcarbazole) (PVK). In an alternativeembodiment, the emitter is part of the repeating unit of the polymer,for example Dow's polyfluorene materials. Both fluorescent andphosphorescent emitters may be appended to polymer chains and used tomake OLEDs. Layers in a device comprising a polymeric matrix aretypically deposited by spin-coating.

Preferred host matrix materials for the emissive layer include carbazolebiphenyl (CBP), and derivatives thereof, N,N′-dicarbazoloylbenzenes, andderivatives thereof, and N,N′,N″-1,3,5-tricarbazoloylbenzenes, andderivatives thereof. Derivatives may include the above compoundssubstituted with one or more alkyl, alkenyl, alkynyl, aryl, CN, CF₃,CO₂alkyl, C(O)alkyl, N(alkyl)₂, NO₂, O-alkyl, and halo. Particularlypreferred host matrix materials for the emissive layer include4,4′-N,N′-dicarbazole-biphenyl (CBP), N,N′-meta-dicarbazoloylbenzene(mCP), and N,N′,N″-1,3,5-tricarbazoloylbenzene (tCP). CBP has a numberof important properties as a matrix material, such as, a high tripletenergy of 2.56 eV (484 nm) and ambipolar charge transporting properties,that make it an excellent host for phosphorescent dopants.

Neat thin films of CBP crystallize readily. Doping a small molecule intothe CBP (such as an emissive dopant) stabilizes the film in an amorphousor glassy form, which is stable for long periods. mCP on the other handforms a stable glass, even when undoped. Crystallization during deviceoperation can lead to failure of the device, and so is avoided. Having amaterial that forms a stable glass, even when undoped, is a benefit,since the crystallization process is less likely to occur. A metric thatis used to evaluate glass forming ability of a given material is itsglass transition temperature, T_(g). This temperature characterizes thethermal stability of a glassy material, thus a high T_(g) is desirablefor OLED materials. At the T_(g), a significant thermal expansiontypically occurs, leading to device failure. mCP has a T_(g) value of65° C. While this value is acceptable for device preparation, a higherT_(g) may be desirable for the making devices with the longest possiblelifetimes. Increasing the T_(g) is readily accomplished by adding large,rigid groups to molecules, such as phenyl and poly-phenyl groups, andsimilar aryl groups. This phenyl addition/substitution should be done ina manner that does not lower the triplet energy, however, or the hostmaterial will not be as suitable for use in blue or white emissivedevices. For example, adding phenyl groups to the carbazole unit itself(e.g. 4′ positions in the Compound I) will generally lower the tripletenergy, making the mCP derivative less suitable for blue or whitedevices.

Substitution of phenyl or poly-phenyl groups at the 2, 4, 5 or 6position in Compound I will most likely not lead to significant shiftsin the triplet energy. These substitutions will generally increase theT_(g) of the materials, making them better materials for long livedOLEDs. Examples of such compounds include, but are not limited to:

and derivatives thereof.

Suitable electrode (i.e., anode and cathode) materials includeconductive materials such as a metal, a metal alloy or an electricallyconductive oxide such as ITO, which are connected to electricalcontacts. The deposition of electrical contacts may be accomplished byvapor deposition or other suitable metal deposition techniques. Theseelectrical contacts may be made, for example, from indium, magnesium,platinum, gold, silver or combinations such as Ti/Pt/Au, Cr/Au or Mg/Ag.

When depositing the top electrode layer (i.e., the cathode or the anode,typically the cathode), that is, the electrode on the side of the OLEDfurthest from the substrate, damage to the organic layers should beavoided. For example, organic layers should not be heated above theirglass transition temperature. Top electrodes are preferably depositedfrom a direction substantially perpendicular to the substrate.

The electrode that functions as the anode preferably comprises high workfunction metals (≧4.5 eV), or a transparent electrically conductiveoxide, such as indium tin oxide (ITO), zinc tin oxide, or the like.

In preferred embodiments, the cathode is preferably a low work function,electron-injecting material, such as a metal layer. Preferably, thecathode material has a work function that is less than about 4 electronvolts. The metal cathode layer may be comprised of a substantiallythicker metal layer if the cathode layer is opaque. If the cathode isintended to be transparent, a thin low-work function metal may be usedin combination with a transparent electrically conductive oxide, such asITO. Such transparent cathodes may have a metal layer with a thicknessof 50-400 Å, preferably about 100 Å. A transparent cathode, such asLiF/Al may also be used.

For top-emitting devices, a transparent cathode such as disclosed inU.S. Pat. No. 5,703,436, or co-pending patent applications U.S. Ser. No.08/964,863, now U.S. Pat. No. 6,469,437 and Ser. No. 09/054,707, nowU.S. Pat. No. 6,420,031, each incorporated herein by reference, may beused. A transparent cathode has light transmission characteristics suchthat the OLED has an optical transmission of at least about 50%.Preferably, the transparent cathode has light transmissioncharacteristics that permit the OLED to have an optical transmission ofat least about 70%, more preferably, at least about 85%.

The devices of the present invention may comprise additional layers,such as an exciton blocking layer (EBL), a hole blocking layer (HBL) ora hole injection layer (HIL). One embodiment of the invention uses anexciton blocking layer that blocks exciton diffusion so as to improveoverall device efficiency, such as disclosed in U.S. Pat. No. 6,097,147,which is incorporated herein in its entirety by reference.

To prevent electron or exciton leakage from the luminescent layer intothe hole transporting layer, particularly in devices having a highenergy (blue) phosphorescent emitter, an electron/exciton blocking layermay be included between the luminescent layer and the HTL. High energyphosphorescent dopants tend to have high energy LUMO levels, approachingthose of the transport and host materials. If the dopant LUMO levelapproaches the LUMO energy of the HTL material, electrons can leak intothe HTL. Likewise, exciton leakage into the HTL layer can occur as theemission energy of the dopant approaches the absorption energy of HTLmaterial. Therefore, introduction of an electron/exciton blocking layerbetween the HTL and luminescent layer may improve the devicecharacteristics. An efficient electron/exciton blocking material willhave a wide energy gap to prevent exciton leakage into the HTL, a highLUMO level to block electrons, and a HOMO level above that of the HTL. Apreferred material for use in an electron/exciton blocking layer is thefac-tris(1-phenylpyrazolato-N,C^(2′))iridium(III) (Irppz).

In still other embodiments of the invention, a hole injecting layer maybe present between the anode layer and the hole transporting layer. Thehole injecting materials of the present invention may be characterizedas materials that planarize or wet the anode surface so as to provideefficient hole injection from the anode into the hole injectingmaterial. The hole injecting materials of the present invention arefurther characterized as having HOMO energy levels that favorably matchup, as defined by their relative IP energies, with the adjacent anodelayer on one side of the HIL layer and the emitter-doped electrontransporting layer on the opposite side of the HIL. The highest occupiedmolecular orbital (HOMO) obtained for each material corresponds to itsionization potential (IP). The lowest unoccupied molecular orbital(LUMO) is equal to the IP plus the optical energy gap, as determinedfrom absorption spectra. Relative alignments of the energies in thefully assembled devices may differ somewhat from those predicted, forexample from the absorption spectra.

The HIL materials, while still being hole transporting materials, aredistinguished from conventional hole transporting materials that aretypically used in the hole transporting layer of an OLED in that suchHIL materials have a hole mobility that may be substantially less thanthe hole mobility of conventional hole transporting materials. Forexample, m-MTDATA has been identified as effective in promotinginjection of holes from ITO into the HTL consisting of, for exampleα-NPD or TPD. Possibly, the HIL effectively injects holes due to areduction of the HTL HOMO level/ITO offset energy, or to wetting of theITO surface. The HIL material m-MTDATA is believed to have a holemobility of about 3×10⁻⁵ cm²/V sec as compared with a hole mobility ofabout 5×10⁻⁴ cm²/V sec and 9×10⁻⁴ cm²/V sec of α-NPD and TPD,respectively. Thus, the m-MTDATA material has a hole mobility more thanan order of magnitude less than the commonly used HTL materials α-NPDand TPD.

Other HIL materials include phthalocyanine compounds, such as copperphthalocyanine, or still other materials, including polymeric materialssuch as poly-3,4-ethylenedioxythiophene (PEDOT) orpoly(ethene-dioxythiophene):poly(styrene sulphonic acid) (PEDOT:PSS)which are effective in promoting injection of holes from the anode intothe HIL material and subsequently into the HTL.

The thickness of the HIL of the present invention needs to be thickenough to help planarize or wet the surface of the anode layer. Forexample, an HIL thickness of as little as 10 nm may be acceptable for avery smooth anode surface. However, since anode surfaces tend to be veryrough, a thickness for the HIL of up to 50 nm may be desired in somecases.

Substrates according to the present invention may be opaque orsubstantially transparent, rigid or flexible, and/or plastic, metal orglass. Although not limited to the thickness ranges recited herein, thesubstrate may be as thin as 10 mm if present as a flexible plastic ormetal foil substrate, or substantially thicker if present as a rigid,transparent or opaque substrate, or if the substrate is made of silicon.

The OLEDs and OLED structures of the present invention optionallycontain additional materials or layers depending on the desired effect;such as protective layers (to protect certain materials during thefabrication process), insulating layers, reflective layers to guidewaves in certain directions, and protective caps, which cover theelectrodes and organic layers in order to protect these layers from theenvironment. A description of insulating layers and protective caps iscontained for example, in U.S. Pat. No. 6,013,538, which is incorporatedherein by reference.

Although a high CRI value is often preferred, the devices of the presentinvention may be used to produce a light source that provides othercolors as well. Incandescent bulbs are actually slightly yellow, ratherthan pure white. By changing the ratio of monomer emitter to aggregateemitter, as described herein, the color of the resulting device can betuned, for example, to imitate the light emitted from an incandescentbulb. By adjusting the concentration of a dopant, the steric bulk of adopant, and the host material used in the emissive layer, a device maybe constructed that will provide an unsaturated (not monochromatic)colored emission.

There may be substantial variation of the type, number, thickness andorder of the layers that are present, dependent on whether an invertedsequence of OLED layers is present, or whether still other designvariations are used. Those with skill in the art may recognize variousmodifications to the embodiments of the invention described andillustrated herein. Such modifications are intended to be covered by thespirit and scope of the present invention. That is, while the inventionhas been described in detail with reference to certain embodiments, itwill be recognized by those skilled in the art that there are otherembodiments of the invention within the spirit and scope of the claims.

EXAMPLES

Where available, solvents and reagents were purchased from AldrichChemical Company. The reagents were of the highest purity and used asreceived.

The ligand 2-(2,4-difluorophenyl)pyridine (F₂ppy) was prepared by Suzukicoupling 2,4-difluorophenylboronic acid and 2-bromopyridine (Aldrich).The Pt(II) μ-dichloro-bridged dimer [(F₂ppy )₂Pt(μ-Cl)₂Pt(F₂ppy)₂] wasprepared by a modified method of Lewis. (Lohse, O. et al. Synlett. 1999,1, 45-48). The dimer was treated with 3 equivalents of the chelatingdiketone ligand and 10 equivalents of Na₂CO₃.2,6-dimethyl-3,5-heptanedione, and 6-methyl-2,4-heptanedione werepurchased from TCI. 3-ethyl-2,4-pentandione was purchased from Aldrich.The solvent was removed under reduced pressure, and the compoundpurified chromatographically. The product was recrystallized fromdichloromethane/methanol and then sublimed.

Irppz was prepared by dissolving Ir(acac)₃ (3.0 g) and 1-phenylpyrazole(3.1 g) in 100 ml glycerol and refluxing for 12 hours under and inertatmosphere. After cooling the product was isolated by filtration andwashed with several portions of distilled water, methanol, ether andhexanes and then vacuum dried. The crude product was then sublimed in atemperature gradient of 220-250° C. to give a pale yellow product (yield58%)

mCP was prepared by the palladium-catalyzed cross coupling of arylhalides and arylamines. (T. Yamamoto, M. Nisbiyama, Y. Koie Tet. Lett.,1998, 39, 2367-2370).

Example 1

The electrophosphorescent excimer WOLEDs were grown on a glass substratepre-coated with an indium-tin-oxide (ITO) layer having a sheetresistance of 20-W/sq. Prior to organic layer deposition, the substrateswere degreased in ultrasonic solvent baths and then treated with anoxygen plasma for 8 min. at 20 W and 150 mTorr.Poly(ethylene-dioxythiophene):poly(styrene sulphonic acid) (PEDOT:PSS),used to decrease OLED leakage current and to increase fabrication yield,was spun onto the ITO at 4000 rpm for 40 s, and then baked in vacuum for15 min at 120° C., attaining an approximate thickness of 40 nm. The holetransporting and host materials, as well as the two dopants wereprepared by standard procedures (See Lamansky, S. et al., Inorg. Chem.40, 1704-1711, 2001) and purified by thermal gradient vacuumsublimation. The molecular organic layers were sequentially depositedwithout breaking vacuum by thermal evaporation at a base pressure of<8×10⁻⁷ Torr.

Deposition began with a 30 nm-thick4,4′-bis[N-(1-naphthyl)-N-phenyl-amino]biphenyl (α-NPD) hole transportlayer (HTL), followed by a 30 nm thick emission region consisting of theblue emitting phosphors FIr(pic) and FPt(acac), both doped at 6 wt %into a 4,4′-N,N′-dicarbazole-biphenyl (CBP) host. The final organiclayer deposited was 50 nm of bathocuproine (BCP). This layer serves as ahole and exciton blocking layer, and as an electron transport medium.

After deposition of the organic layers, the samples were transferredfrom the evaporation chamber into a N₂ filled glove box containing ≦1ppm of H₂O and O₂. After affixing masks with 1 mm diameter openings tothe samples, they were transferred into a second vacuum chamber(<10⁻⁷Torr) where the cathode metal (consisting of 5 Å of LiF followedby 70 nm of Al) was deposited through the masks. The samples were onlyexposed to air while being tested. A cross section of the devicestructure is shown in FIG. 6.

It is convenient to begin the design of the WOLED by examining thephotoluminescence emission (PL) and excitation (PLE) spectra of thematerials used in the device emissive region. Three doped films and anundoped “control” CBP film, each 1000 Å thick, were grown by thermalevaporation on separately solvent-cleaned quartz substrates. The PL andPLE spectra of the films were taken using a Photon TechnologyInternational QuantaMaster fluorescence system. FIG. 5 lists thecomposition of the films and their associated PL CIE coordinates.

FIG. 5 shows the PL (solid lines) and PLE (open circles) spectra of thefilms 1-4. Film 1 shows the CBP PL spectrum with a peak at λ=390 nm andthe corresponding PLE between λ=220- and 370 nm. The PLE of CBP has ashoulder at λ=300 nm and a main peak at λ=350 nm. The CBP PLE peakscorrespond to the absorption peaks at λ=300- and 350 nm (arrows, insetof FIG. 6). These two CBP features appear in the PLE spectra of allfilms where CBP was used as a host; therefore, it is the main absorbingspecies in all the films, and energy must be transferring efficientlyfrom CBP to both FPt(acac) and FIr(pic) for emission from thesemolecules to occur.

The PL spectrum of Film 2 shows bands consistent with CBP and FPt(acac)monomer emission only. CBP emission is at λ=390 nm, and FPt(acac)monomeric emission has peaks at λ=470 nm and λ=500 nm (see FIG. 5). Thespectrum observed for FPt(acac) in CBP is very similar to the samemolecule in dilute solution. At <1 wt %, the randomly distributedFPt(acac) molecules are, on average, separated by 30 Å, precludingsignificant excimer formation. The lack of a broad, long wavelength peakin Film 2 suggests that exciplexes do not form between CBP andFPt(acac). That is, if exciplexes form between these moieties, exciplexemission from a FPt(acac)-CBP complex would be present even in the mostlightly doped samples.

As the FPt(acac) doping concentration is increased to ˜7 wt % (Film 3),strong excimer emission is observed with an orange-red peak at λ=570 nmalong with the characteristic monomer emission at λ=470 nm and λ=500 nm.The higher doping level leads to complete quenching of the CBPfluorescence. For Film 3, the measured lifetime of t=7.2 ms of theFPt(acac) emission at λ=570 nm, compared with 8.3 ms at λ=470 nm, isalso consistent with excimer formation on FPt(acac) complexes.

Doping of FPt(acac) between 1 wt % and 7 wt % leads to spectra that areconsistent with simultaneous monomer and excimer emission from thedopant. At a doping level of 3 wt %-4 wt % the monomer and excimer linesare balanced, leading to white emission. While this film compositioncould in principal be used to make a white OLED, the doping level is toolow for the device to have a reasonable efficiency and give a spectrumfree of CBP fluorescence.

Film 4 consists of CBP doped with 6 wt % FIr(pic) and 6 wt % FPt(acac).Here, CBP emission is absent in the PL, but the PLE spectrum stillindicates that it is the main absorbing species, and that energy isefficiently transferred to both FIr(pic) and FPt(acac). The PL emissionof the double-doped film is similar to the electroluminescence (EL) ofthe WOLED, shown in FIG. 6.

The energy transfer process can be understood in the double-doped systemby referring to the highest occupied molecular orbital (HOMO) and lowestunoccupied molecular orbital energies (LUMO) of the three organiccomponents (lower right inset, FIG. 7). The transfer of triplet energyfrom CBP to FIr(pic) occurs via an endothermic process described byAdachi, C. et al., Appl. Phys. Lett. 79, 2082-2084 (2001). Assuming thesame positions of the HOMO energy levels of (5.8±0.1 eV), and the LUMOenergy levels of (3.2±0.1 eV) of both FPt(acac) and FIr(pic), a similarendothermic triplet energy transfer pathway can be expected for CBP andFPt(acac). Resonant energy transfer between the triplet levels of thetwo dopants is also likely since they are present at high concentrationin the CBP matrix. However, direct energy transfer from FIr(pic) to theexcimer can not occur because the excimer has zero ground stateabsorption, preventing the cascading of energy from the blue to theyellow emission centers. This essentially decouples the excited statesof these molecules, allowing for simple optimization of the doping toachieve the desired color balance.

The light output power from the WOLED was measured using a Newport PowerMeter and a calibrated silicon photo diode, and then calculated η_(ext)using current density-voltage characteristics shown in the left inset ofFIG. 7. A Lambertia intensity profile is assumed to calculate η_(p)(FIG. 7) and luminance. Here, η_(ext)≧3.0% between J=×10⁻³ mA/cm² and 10mA/cm². The roll-off at J>300 mA/cm² is attributed to sample heating andtriplet-triplet annihilation. The WOLED has a maximum η_(ext)=(4.0±0.4)%corresponding to (9.2±0.9)cd/A, a luminance of (31000±3000)cd/m² at 16.6V, η_(p)=(4.4±0.4)lm/W and a CRI of 78.

A neat film of FPt(acac) has measured lifetimes of t=4.8 ms and 5.2 msat λ=470 nm and λ=600 ms, respectively. Hence, as the current densityincreases, the FPt(acac) excimer states may become saturated compared tothe monomer and FIr(pic) and lead to increased blue emission. Thespectral changes reflect small changes in CIE coordinates from (0.40,0.44) to (0.35, 0.43).

Example 2

OLEDs were prepared with FPt3 at doping levels of 8, 10 and 12%. Thedevice structure consisted of ITO/NPD (400 Å)/Ir(ppz)₃ (200 Å)/CBP-FPt3(300 Å)/BCP (150 Å)/Alq₃ (200 Å)/Mg-Ag. The current voltagecharacteristics of the-three devices were similar, with progressivelyless leakage current at low voltage as the doping level is increased.The CBP host emission was not observed at any of the doping levels,indicating that the FPt3 dopant is efficiently trapping all of theexcitons formed in the CBP matrix. While exciton formation in the CBP isa possible result of hole-electron recombination, it is also possiblethat the hole or electron could be trapped at the FPt3 molecule anddirect recombination at the dopant occurs. The latter process will leadto excitons being formed on the dopant, without requiring energytransfer from the matrix material, i.e. CBP in this case. The Ir(ppz)₃electron blocking layer was necessary to prevent electron leakage intothe NPD layer, which leads to NPD emission in addition to dopantmonomer-excimer emission. The three devices all show turn-on voltagebetween 3 and 4 volts and achieve maximum luminances between 4,000 and10,000 Cd/m². The spectra of the devices show very little variation asthe voltage is raised, i.e. the monomer to excimer ratio is notsignificantly affected by voltage or current density. The 10 and 12%doped devices give peak efficiencies of 4 and 3.5%, respectively. The 10and 12% doped devices also give very good power efficiencies, of 8 and6.5 lm/W, respectively (at 1 Cd/m²).

Example 4

Thin films of mCP doped with varying wt % of FPt were prepared byco-depositing the, two materials onto a glass substrate. The spectra ofFPt doped in mCP, at a range of concentrations, are shown in FIG. 20.The wavelengths of the emission maxima for the monomer and aggregatestates of FPt doped into mCP are the same as those of FPt in CBP.Balanced monomer/aggregate emission is observed at a doping level ofapproximately 15 wt %, roughly three times the concentration required toachieve an equivalent monomer/aggregate emission ratio from FPt dopedCBP films. This suggests that mCP is a better solvent for FPt, leadingto fewer FPt . . . FPt interactions in the doped mCP film, at a givenconcentration.

In contrast to the CBP doped films, no host emission is observed in thephotoluminescence spectra of lightly doped mCP films (<1 wt % FPt),indicating that energy transfer from mCP to FPt is more efficient thanfrom CBP to FPt. Despite the high triplet energy of CBP (phosphorescenceλ_(max)=460 nm), energy transfer from CBP to blue phosphorescentdopants, such as the Pt complexes used here, is an endothermic process.In contrast, mCP has a phosphorescence spectrum peaked at 410 nm, makingenergy transfer from mCP to the Pt complex dopants a more efficient,exothermic process. A more efficient energy transfer from the host tothe dopant will affect the amount of dopant necessary to quenchemission, as observed.

Both CBP and mCP have low dipole moments (ca. 0.5 D), so electrostaticinteractions between the dopants and host materials are expected to besimilar. This is consistent with the observation that the spectra ofmonomer and aggregate states for doped mCP and CBP films are the same.Without being limited by theory, the differences between CBP and mCP,which give rise to differing dopant solubilities, is related to theirmolecular structures. Planar molecules tend to have high associationenergies, which promote crystallization and hinder glass formation. CBPis expected to be largely planar in the solid state. This is consistentwith our observation that undoped CBP thin films rapidly crystallizewhen deposited directly on glass or ITO substrates. The high CBPassociation energy may tend to exclude monomer dopant, leading toaggregate formation at moderate doping levels. mCP readily forms astable glass when deposited on either inorganic or organic substrates,suggesting it has a nonplanar ground state structure. The glasstransition temperatures for the mCP is 65° C. Steric interactionsbetween adjacent carbazole groups and the phenyl ring lead to aprediction that both CBP and mCP should have nonplanar ground statestructures, as seen in the geometry of the energy minimized structuresin FIG. 21. While the minimized structure of CBP appears somewhatnonplanar, it is important to note that the calculated energy differencebetween the structure shown and the planar conformer is only 18 kJ/mol.In contrast, the energy cost to planarize mCP is 35 kJ/mol. Theprincipal cause of the large barrier to flatten mCP is H . . . Hrepulsions between adjacent carbazoles, interactions that are absent inCBP. Based on the structural differences, we expect the degree ofsolvation of a square planar Pt dopant by mCP to be very different fromthat of CBP. This change significantly affects the monomer/aggregateratio at a given doping level in CBP vs. mCP.

The CIE coordinates and the color rendering index (CRI) for thephotoluminescence spectra of 1 doped into mCP are given in Table 1.

TABLE 1 Concentration, weight % CIE X CIE Y CRI 0.1% 0.15 0.28 — 0.5%0.19 0.32 — 4.5% 0.21 0.35 44.5  10% 0.27 0.39 59.7  15% 0.32 0.41 68.3 20% 0.32 0.39 73.2  25% 0.41 0.46 64.1  30% 0.41 0.45 69.2Concentrations between 4-10 wt % gave the CIE coordinates closest towhite (0.33, 0.33) while the maximum CRI was observed for concentrationsranging between 15-20 wt %. At the higher concentrations, the CIEcoordinates are close to those found in incandescent lamps (ca. 0.41,0.41). Therefore, the 10-20 wt % concentration range for 1 doped mCP waschosen to be optimal for use in WOLEDs.

Example 5

A device was fabricated with the structure NPD (400 Å)/Irppz(200Å)/mCP:FPt (16% 300 Å)/BCP (150 Å)/Alq3(200 Å)/LiF(10 Å)/Al(1000 Å). Theuse of the mCP host in place of CBP significantly improves the deviceperformance. The efficiency, current-voltage characteristics, andspectra of the device are shown in FIGS. 23, 24 and 25. The higherdoping concentrations and improved energy transfer from mCP to thedopant gave a maximum quantum efficiency of 6.4±0.6% (12.2±1.4 lum/W,17.0 cd/A) at low brightness levels (1 cd/m²) and 4.3±0.5% (8.1±0.6lum/W, 11.3 cd/A) at 500 cd/m². The quantum efficiencies demonstrated bythese mCP/FPt WOLEDs are the highest reported efficiencies for a WOLED.The quantum efficiency decreases with increasing current density, asobserved for other devices, however, the decrease is less severe thanmost other electrophosphorescent devices.

Example 6

An OLED was prepared as in Example 3 except the Irppz EBL was omitted(i.e. NPD/mCP-FPt/BCP/Alq3). The EL spectrum has a significantcontribution from NPD and quantum efficiency of the devices drops byroughly a factor of two. (see FIG. 24). Overall, the Irppzelectron/exciton blocking layer increases the OLED efficiency, removesNPD emission from the spectra and makes the spectrum independent ofvoltage.

The energy level diagram for the mCP and CBP devices, shown in FIG. 22,illustrates that the barrier for migration of electrons from thedopant/CBP LUMO levels to the NPD LUMO may be comparable to the holeinjection barrier from NPD into the emissive layer. Eliminatingelectron/exciton leakage into the HTL should improve both the WOLEDefficiency and color stability. The Irppz complex emits exclusively froma phosphorescent excited state (λ_(max)=414 nm at 77K, τ=15 μsec). Theoptical gap for this complex was taken as the low energy edge of theabsorption spectrum, at 370 nm (3.4 eV). This estimate of the opticalgap represents a lower limit for the carrier gap. Irppz shows areversible oxidation in fluid solution at 0.38 V (vs.ferrocene/ferrocenium), but no reduction wave occurs out to −3.0V inDMF, consistent with a carrier gap of >3.4 eV. The HOMO energy for Irppzwas measured by Ultraviolet Photoelectron Spectroscopy (UPS) and foundto be 5.5 eV. Using the Irppz optical gap to approximate the carriergap, we estimate the Irppz LUMO is 2.1 eV, well above both the CBP anddopant LUMOs. The energy scheme of FIG. 22 suggests that Irppz shouldmake an excellent electron/exciton blocking layer.

While the present invention is described with respect to particularexamples and preferred embodiments, it is understood that the presentinvention is not limited to these examples and embodiments. Inparticular, the present invention may be applied to a wide variety ofelectronic devices. The present invention as claimed therefore includesvariations from the particular examples and preferred embodimentsdescribed herein, as will be apparent to one of skill in the art.

1. An organic light emitting device comprising an emissive layer,wherein the emissive layer comprises an aggregate emitter, a monomeremitter, and a matrix material comprising a compound of the formula I

wherein the substituents at the 2, 4, 5, 6, and each 4′ position areindependently selected from hydrogen, phenyl, or polyphenyl; and whereinthe emission from the aggregate emitter is lower in energy than theemission from the monomer emitter, and wherein the combined emission ofthe aggregate emitter and the monomer emitter sufficiently spans thevisible spectrum to give a white emission.
 2. The device of claim 1,wherein the aggregate emitter is an excimer.
 3. The device of claim 1,wherein the aggregate emitter and the monomer emitter emit byphosphorescence.
 4. The device of claim 3, wherein the aggregate emitterand the monomer emitter are comprised of the same chemical compound. 5.The device of claim 3, wherein the monomer emitter and aggregate emitterare phosphorescent organometallic compounds.
 6. The device of claim 1,wherein the combined emission has a color rendering index of at leastabout
 80. 7. The device of claim 1, wherein the combined emission has aCIE x-coordinate of about 0.30 to about 0.40 and a CIE y-coordinate ofabout 0.30 to about 0.45.
 8. The device of claim 1, wherein the emissivelayer comprises an exciplex emitter and a monomer emitter.
 9. The deviceof claim 8, wherein the exciplex emitter and the monomer emitter emit byphosphorescence.
 10. The device of claim 9, wherein the monomer emitterand exciplex emitter are phosphorescent organometallic compounds.
 11. Anorganic light emitting device comprising an anode; a hole transportinglayer; an emissive layer comprising an aggregate emitter, a monomeremitter, and a matrix material comprising a compound of the formula I

wherein the substituents at the 2, 4, 5, 6, and each 4′ position areindependently selected from hydrogen, phenyl, or polyphenyl; and anelectron transporting layer; and a cathode; wherein the emission fromthe aggregate emitter is lower in energy than the emission from themonomer emitter, and wherein the combined emission of the aggregateemitter and the monomer emitter sufficiently spans the visible spectrumto give a white emission.
 12. The device of claim 11, wherein theemissive layer comprises an excimer emitter and a monomer emitter. 13.The device of claim 11, wherein the aggregate emitter and the monomeremitter emit by phosphorescence.
 14. The device of claim 13, wherein theaggregate emitter and the monomer emitter are comprised of the samechemical compound.
 15. The device of claim 13, wherein the aggregateemitter and the monomer emitter are phosphorescent organometalliccompounds.
 16. The device of claim 11, wherein the combined emission hasa color rendering index of at least about
 80. 17. The device of claim11, wherein the combined emission has a CIE x-coordinate of about 0.30to about 0.40 and a CIE y-coordinate of about 0.30 to about 0.45. 18.The device of claim 11, wherein the aggregate emitter is an exciplex.19. The device of claim 18, wherein the exciplex emitter and the monomeremitter emit by phosphorescence.
 20. The device of claim 19, wherein themonomer emitter and exciplex emitter are phosphorescent organometalliccompounds.
 21. The device of claim 11, wherein the device furthercomprises an exciton blocking layer.
 22. The device of claim 21, whereinthe exciton blocking layer is situated between the hole transportinglayer and the emissive layer.
 23. The device of claim 22, wherein theexciton blocking layer comprises fac-Iridium(III)tris(1-phenylpyrazolato-N,C^(2′)).
 24. The device of claim 1, whereinthe matrix material comprises a compound of the formula


25. The device of claim 1, wherein the matrix material comprises mCP.26. The device of claim 11, wherein the matrix material comprises acompound of the formula


27. The device of claim 11, wherein the matrix material comprises mCP.